Occurrence of Dalbergia obovata in grasslands of urban nature reserves within a metropolitan municipality: is it an encroaching woody species?

Abstract

The occurrence of Dalbergia obovata, which is typically associated with forests, in grasslands within conservation areas in a metropolitan municipality was investigated to assess whether it was encroaching and what the possible causes might be. Relevant legislation has not regarded D. obovata as an encroacher species in grasslands. The abundance and distribution patterns of D. obovata in four conservation areas were mapped in the field and captured as GIS layers to facilitate calculation of the proportion of each grassland area deemed to have been encroached over two decades. Management records for 17 conservation areas were analysed to determine whether there were associations between different management practices and D. obovata encroachment. We found that more than one third of grasslands present in 1999 were lost to encroachment by D. obovata and that some grassland areas were almost completely (99%) encroached. Results of a Chi-squared test and principal components analysis of management records indicated that the absence of large herbivores was significantly associated with D. obovate encroachment into grasslands. We conclude that D. obovata should be listed as a woody encroacher species and that some conventional management practises applied to control woody encroachers, such as the use of fire, are ineffective against D. obovata.

Keywords: fire; herbivory; savanna; tropical grassy biomes; woody encroachment